1. Field of the Invention
The present invention relates to an optical device suitable for recording or reading optical disks such as CD (a compact disk) and DVD (a digital video disk).
2. Discussion of Background
There have been proposed optical devices for recording or reading DVD having a transparent substrate of 0.6 mm thick and CD having a transparent substrate of 1.2 mm thick by means of a single objective lens (JP-A-9-43510, JP-A-9-81953).
In a device disclosed in JP-A-9-43510, a divergence angle changing lens for changing a divergence angle of light from a laser light source is provided between the laser light source and an objective lens, and when DVD or CD is recorded or read, the divergence angle changing lens is moved along the optical axis depending on a thickness of a transparent substrate of DVD or CD whereby aberration is corrected.
In a device disclosed in JP-A-9-81953, a finite type objective lens is commonly used for DVD and CD, and when recording or reading is effected for DVD or CD, a laser light source is moved along the optical axis in response to a thickness of a transparent substrate of DVD or CD whereby aberration is corrected.
In the former device, when switching of mode is conducted from the recording or reading of DVD to the recording or reading of CD, aberration characteristics of the lens concerning light passing on the optical axis, in particular, a spherical aberration can be made substantially zero by moving the divergence angle changing lens along the optical axis. However, it required a mechanism to move the divergence angle changing lens. Further, aberration characteristics on light passing out of the optical axis, in particular, an off-axial coma aberration could not be satisfied.
In the later device, when switching of mode is conducted from the recording or reading of DVD to the recording or reading of CD, aberration characteristics of light passing on the optical axis, in particular, a spherical aberration can be made substantially zero by moving the light source along the optical axis to thereby change an object-image distance. However, this mechanism requires a mechanism for changing the object-image distance. Further, aberration characteristics on light passing out of the optical axis, in particular, an off-axial coma aberration could not be satisfied.
Since the above-mentioned conventional devices have a large off-axial coma aberration, the optical performance shows a large reduction when the light source, the objective lens and so on are positioned inclined from the optical axis or shifted from the optical axis. Accordingly, high accuracy was required in positioning the objective lens and other elements in assembling operations of the optical device, and therefore, productivity is decreased. Further, in the conventional device for which high accuracy is required in determination of the position of the objective lens or the other elements, gradual deterioration of the optical performance is caused due to the wearing of a moving mechanism for moving the lens or the light source, the wearing resulting an inclination of the objective lens and other elements from the optical axis or a shift thereof from the optical axis.
Further, since the conventional devices have a large off-axial coma aberration, an allowable range is narrow in determining the position of the objective lens, in particular, a shift from the optical axis in driving the objective lens through an autofocusing mechanism when the optical devices are actually operated. Therefore, the optical performance is reduced. Further, since the conventional optical devices have a moving mechanism, it is difficult to achieve size reduction and weight reduction.